1. Field of the Invention
The present invention relates to a magnetic field measurement system and an optical pumping magnetometer, and more specifically to a magnetic field measurement system for measuring biomagnetism.
2. Description of the Related Art
When performing accurate biomagnetic measurement, it is essential to grasp the degree of environmental magnetic noise (such as geomagnetism or time-varying magnetism generated when an automobile or an electric train passes nearby) that hinder biomagnetic signal measurement. Conventional combinations of a magnetic sensor for biomagnetic measurement and a reference sensor for environmental magnetic measurement, which are used for highly accurate biomagnetic measurement, include a combination of a superconducting quantum interference device (SQUID) magnetometer and a fluxgate magnetometer, a combination of a SQUID magnetometer and another SQUID magnetometer, and a combination of an optical pumping magnetometer and another optical pumping magnetometer.
In the case of using the SQUID magnetometer as the magnetic sensor for the biomagnetic measurement while using the fluxgate magnetometer as the reference sensor for the environmental magnetic measurement, the SQUID magnetometer is housed in a Dewar containing liquid helium (or liquid nitrogen) located inside a magnetic shield room for shielding the environmental magnetic noise. The fluxgate magnetometer is disposed outside the magnetic shield room to measure the environmental magnetic noise of the outside. A voltage signal obtained by the fluxgate magnetometer is converted into an amount of current by a current converter, and the current is caused to flow through a magnetic field generating coil disposed on an outside wall of the magnetic shield room to reduce the environmental magnetic noise that enters the magnetic shield room (Japanese Patent Laid-Open Official Gazette No. 2000-37362).
In the case of using the SQUID magnetometers both as the magnetic sensor for the biomagnetic measurement and as the reference sensor for the environmental magnetic measurement, the respective SQUID magnetometers are housed in the Dewar containing liquid helium (or liquid nitrogen) located inside the magnetic shield room. The SQUID magnetometer for the biomagnetic measurement is located immediately above a measurement object, and the SQUID magnetometer as the reference sensor is located above the SQUID magnetometer for the biomagnetic measurement. A difference between signals obtained from the respective SQUID magnetometers is calculated by use of a subtraction circuit to detect only a biomagnetic signal coming out of the measurement object (Japanese Patent Laid-Open Official Gazette No. Hei-11(1999)-309122).
In the case of using the optical pumping magnetometers both as the magnetic sensor for the biomagnetic measurement and as the reference sensor for the environmental magnetic measurement, a vapor cell for the biomagnetic measurement is located immediately above a measurement object, and a vapor cell as the reference sensor is located above the vapor cell for the biomagnetic measurement. A difference between output signals from lock-in amplifiers, the signals obtained by the respective optical pumping magnetometers, is calculated by use of a subtraction circuit to detect only a biomagnetic signal coming out of the measurement object (Appl. Phys. B76, pp. 325-328 (2003)).
In addition, a method is also reported in which a vertical cavity surface emitting semiconductor laser is used as a chip-scale small optical pumping magnetometer (Appl. Phys. Lett. 85, 6409 (2004)). A coherent population trapping (CPT) resonance is applied to the method of using the vertical cavity surface emitting semiconductor laser. The report says that the sensitivity is 50 pT/√Hz, and that it is necessary to improve the sensitivity at least 100 times or more in order to measure biosignals.